Photographic compositions and elements



United States Patent 3,155,517 PHQTOGRAPHIC COMPGSITIQN AND ELEMENTS Ralph Kingsley Blake, Westfield, N..l., assignor to E. ll. du Pont de Nemours and Company, Wilmington, Deh, a corporation of Delaware I No Drawing. Filed Nov. 8, I962, Ser. No. 236,419 11 Qlaims. (iii. 96-407) This invention relates to photography, and more particularly to new photographic products useful therein.

The principal processes of photography are based on the use of colloid-silver halide emulsion layers. In the prior art processes a latent image is formed by imagewise exposure of a radiation-sensitive silver halide emulsion layer. Silver halide bearing a latent image has been developed to silver by selective reduction in these instances.

In the prior processes of photography the unreduced silver remaining after development has been removed by silver halide solvents or rendered insensitive or transparent by treatment with complexing agents. Optional aftertreatments include intensification and reduction, toning and tinting. However, the primary or first step in image formation always has been based on the selective reduction step.

* It is an object of this invention to provide new photographic silver halide compositions, photographic layers and photographic elements bearing a layer of silver halide.

Another object is to provide processes for making these products. A further object is to provide such products which are adapted to more versatile processes for forming silver and other images and which are simple, dependable and give results equal in quality to the prior conventional methods. Still further objects will be apparent from the following description of the invention.

The objects of this invention are realized by new silver halide compositions in which an image may be formed, after exposure to actinic radiation, by imagewise solution of the silver halide. The residual silver halide image may then be converted to silver, dyed or toned images.

The novel photographic compositions and silver halide layers of this invention comprise silver halide crystals which have been treated, in substantially greater than foginhibiting amounts, with a heterocyclic organic nitrogen compound having in the state of its free base at least one intracyclic radical represented by the formula at least one of the bonds being attached to intracyclic carbon, and Whose silver salt is less soluble in water than said silver halide crystals. in these compounds any divalent atom thereof, e.g., sulfur, is in the heterocyclic ring and hence acyclic salt-forming groups (e.g., SH groups) are not present in the heterocyclic organic nitrogen compounds. Tautomeric forms of the organic compound may result in different linkages. The heterocyclic nitrogen compound is characterized in that when admixed With an aqueous silver halide dispersion it protects the silver halide grains to such an extent that when a silver halide dispersion protected by said compound is treated with 10%, by weight, aqueous sodium thiosuliate, at least three times the amount of silver halide remains undissolved, as in a similar dispersion successively treated with aqueous sodium hypochlorite and aqueous sodium thiosulfate, after vigorous agitation of the dispersions for 30 seconds at 25 C. The dispersion so successively treated clears, Whereas the other dispersion remains turbid.

Preferably, the silver halide crystals are dispersed in a water-permeable organic colloid to form a light-sensitive photographic emulsion. The selected nitrogen-containing organic compound can be added to the silver halide emulsion while the latter is in the liquid state or the emulsion may be coated on a suitable support and the resulting element bathed or impregnated with a solution, e.g., an alcoholic solution of the organic compound. In the Working examples below, the amount of organic compound in the silver halide emulsion is from about 2.7 to 250 g. per mole of silver halide, but Wider ranges of concentration can be useful, depending upon the particular organic compound, the size and nature of the silver halide crystals, the presence of other materials which may partially cover the surface of the silver halide crystal, and upon various other factors.

The gelatinzsilver halide ratio is quite flexible and may vary from 3:1 to 1:30 depending on the particular organic compound and application.

In one commercially practical aspect of the invention, the silver halide is present in much higher concentration than in conventional emulsions and emulsion layers.

in an important use of the products of the invention, direct positive images are formed by a process which comprises (a) Exposing imagewise to actinic light a photosensitive layer comprising silver halide crystals treated with the organic compound as described above.

(b) Treating the exposed layer in a solution or" a silver halide solvent to remove soluble silver halide in the exposed image areas, thus forming a positive silver halide image, and

(c) Washing the resulting layers.

If desired, the silver image may be viewed directly, e.g., by projection (if on a transparent support) or it may be intensified by (d) Converting the residual silver halide to silver by treatment in a fogging developer, e.g., a high pH, l-phenyl-4-methyl-3-pyrazolidone/hydroquinone developer containing iodide ion or by fogging the emulsion by exposure to light and then treating with a silver halide reducing agent, e.g., a conventional silver halide developer, and

(2) Washing the developed layer to reveal a positive silver image in the original non-exposed areas.

The imagewise solution of the exposed silver halide/ organic compound stratum may be ellected by the silver halide solvents commonly used as photographic fixing agents, e.g., sodium thiosulfate, sodium thiocyanate, concentrated solutions of potassium bromide, etc. Reduction of the treated, residual silver halide may be accomplished by use of any chemical reducing agent capable of reducing silver ion to silver metal, e.g., hydroquinone, metol, sodium hydrosulfite and stannous chloride. The function of the reducing agent may be enhanced by modifying the surface properties of the treated, residual silver halide crystals by means of alcohol, thiourea, potassium iodide, etc. The silver halide image may be toned, e.g., with sodium sulfide, sodium selenide, etc. In addition, color images may be obtained by developing the treated, residual silver halide with a primary aromatic amine color developing agent in the presence of a color coupling compound either in the developing bath or previously incorporated in the emulsion.

Since a broad new photographic principle has been discovered, the applicant does not wish to be limited to a narrow class of organic nitrogen compounds with which the silver halide crystals may be treated in preparing the novel compositions of this invention. Instead, a number of useful compounds will be shown in the exemplary disclosure and a straightforward and relatively simple test willbe described by which one skilled in the art may readily determine Whether or not other given organic compounds will have utility in this invention. Essentially, the test consists of two parts, Test A and Test B. In Test A, the candidate organic nitrogen compound must render a dispersion of silver halide crystals insoluble in a silver halide solvent, i.e., an aqueous solution of sodium thiesulfate, at some pH between 8 and 13. If the candidate compound effects the insolubility required of Test A, it must also, to have utility, pass Test B by forming with said dispersion of silver halide crystals a reaction product which, upon treatment with an aqueous solution of sodium hypochlorite, becomes soluble when subsequently treated with aqueous sodium thiosulfate. The following practical tests are provided in further exemplification of the invention and include specific concentrations of solutions, times, etc., so that suitable organic compounds may be readily and positively identified.

TEST A A solution nearly saturated at 25 C. with a candidate organic compound is prepared using ethanol, acetone, dimethyl formamide, Water or other suitable solvents. Depending on the solubility, a solution concentration from 0.01 to 10 percent by weight is obtained. Twenty-five ml. of a silver chlorobromide dispersion containing mg. of silver halide (calculated as silver bromide), prepared as described below, is treated with small increments (i.e., about 0.1 to 0.2 ml. at a time) of the said condiclate solution under safelight conditions (Wratten 1A red filter or equivalent) until the silver halide dispersion either is rendered insoluble in 10% aqueous sodium thiosulfate or the candidate is found not to cause insolubili ation. Generally insolubilization will occur upon the addition of 0.05 or less of said candidate compound, calculated as the pure compound. Compounds which must be used in substantially greater quantities than this, e.g., 12 g. to efiect insolubilization are considered less preferred compounds. The silver halide dispersion insolubility is determined by taking a 0.5-ml. portion of the silver halide dispersion (after each incremental addition of the candidate organic compound), adding about 0.1 to 0.2 ml. of 10% aqueous sodium thiosulfate solution and observing the turbidity after seconds.

As a control, one should use 25 ml. of water to which small increments of the candidate solution are added. Half milliliter portions of the control are treated in the same manner with the sodium thiosulfate solution. The presence of visual turbidity relative to the control is sufficient to satisfy the ellnition of insolubility in this test.

This test may be repeated for various pH increments from 3 to 13. Although there is some optimum pH value at which the test is most sensitive, this is not a sharp maximum which must be precisely attained. Rather, it has been found that there is a fairly broad range of pH value (e.g., 2.0 to 3.0 pH units) over which the test has a satisfactory sensitivity. In practice, the silver halide dispersion might be tested, e.g., at pH 9.0 to 10.0 and it insolubilization occurs ere, Test A is completed. if there is no insolubilization, the test is repeated at a pH (e.g., about pH 12). Thus two dilferent pH values represents a practical maximum number which must be investigated to determine whether or not insolubilization will occur.

TEST E An organic compound capable of insolubilizing a silver halide dispersion according to Test A is now ready for the next test, which again will be conducted under safelight conditions. To the above silver halide dispersion, there is added the minimum amount of a solution of the candidate organic compound found necessary for insolubilization. Half milliliter samples of the dispersion containing 0.5 mg. AgBr or 0.29 mg. Ag are placed in two test tubes. To one sample is added 0.5 ml. of water; to the other is added 0.5 ml. of a 5% by weight aqueous solution of sodium hypochlorite (containing 25 mg. sodium hypochlorite). Next. there is added to both samples, 1.0 ml. of an aqueous 10% by weight solution of sodium thiosulfate (containing 100 mg. sodium thiosulfate). If, aiter standing for u to 30 seconds, the sample treated with sodium hypochlorite clarifies (or becomes less turbid) relative to the control sample, the candidate organic compound meets the requirements of Test B and is satisfactory for use as disclosed in this invention.

Silver Halide Dispersion Preparation-Dispersion l The silver halide dispersion disclosed in Tests A and B is prepared according to the following specifications. In red light, 30 g. of photographic grade gelatin is soaked in 1100 mi. of distilled water for 10 minutes. The temperature is then raised to 120 F. and g. of solid ammonium chloride added. The mixture is stirred at F. and after the ammonium chloride is completely dissolved, a solution made by diluting 500 ml. of 3 N silver nitrate with 2000 ml. of distilled water is added while stirring the solution for 5 seconds. This mixture is held at 120 F. for 4 minutes with stirring, and then ml. of 3 N ammonium bromide added (30 mole percent) in 10 seconds. The mixture is held additional 15 minutes at 120 F. with stirring and then cooled to 100 F. A mixture of 75 g. of the sodium salt of technical lauryl alcohol sulfate (a white powder) and 7 ml. of 3 N sulfuric acid is added in 10 seconds to the silver chlorobromide, stirring continued for one minute and then the mixture allowed to settle. The supernatant liquid is decanted and replaced by 2000 mi. of distilled water containing 4 g. of sodium chloride. This mixture is stirred for 5 minutes at 100 F., allowed to settle and decanted again. Two hundred ml. of distilled water is added to the silver halide curds and the temperature adjusted to 95 F. This mixture was vig-' orously stirred for 10 minutes at 95 F. and then the pH adjusted to 6.1101 with aqueous sodium hydroxide solution. The redispersed emulsion is then analyzed for silver halide content calculated as silver bromide and a dispersion made by diluting the appropriate amount with distilled water such that the dispersion contains 1 mg. calculated silver bromide per ml.

Organic compounds useful in this invention and capable of meeting the requirements of Test A and Test B, respectively, may be found among a wide variety of nitrogencontaining compounds and preferred compounds form silver salts which re insoluble in aqueous ammonium hydroxide at pH 12.

Dispersed crystals of silver halide, treated with an appropriate amount of a suitable organic compound are affected by exposure of a portion of said crystals to actinic radiation, e.g., ultraviolet, visible, infrared, X- radiation, etc., to such an extent that at least 20% of the less soluble crystals remain when 90% of the more soluble crystals dissolve when treated in 10% by weight aqueous sodium thiosulfate solution. Generally, solubilizing groups on the heterocyclic nitrogen-containing compound should be avoided in order that reaction prodnets with silver halide will be formed which will significantly reduce the solubility of silver halide crystals in silver halide solvents. There are exceptions to this rule, particularly in the case of suitable compounds containing solubilizing groups which are compensated for by the presence of insolubilizing groups, e.g., long chain alkyl groups. The chemical testing for selecting suitable compounds has been found to give absolute correlation, i.e., organic compounds which have been subjected to the pair of tests have produced Without exception, when tested in actual photographic emulsions, the very elfects predicted by said tests. Combinations of the compounds with various basic dyes, including various cyanine dyes, and Methylene Blue (Colour Index No. 922), Crystal Violet A0 (Colour Index No. 681) and Rhodamine 6 GDN Extra (Colour Index No. 752) have proven use ful additions.

The silver halide need not be a combination of silver chloride and silver bromide, but may be silver chloride, silver bromide and other mixed halide systems conventional in photographic practice, e.g., silver bromoiodide.

'methacrylic esters and amides.

While, for rapid processing, a high silver halide to binder ratio is preferred as described in several of the examples, more conventional ratios can also be used.

In place of part of the gelatin, other natural or synthetic water-permeable organic colloid binding agents can be used and in some cases such binders can be used alone. Such agents include water-permeable or watersoluble polyvinyl alcohol and its derivatives, e.g., partially hydrolysed polyvinyl acetates, ethers and acetals containing a large number of intralinear CH CHOH groups, hydrolysed interpolymers of vinyl acetate and unsaturated addition polymerizable compounds such as maleic anhydride, acrylic and methacrylic acid esters and styrene. Suitable such colloids of the last-mentioned type were disclosed in US. Patents 2,276,322; 2,276,323 and 2,397,866. The useful polyvinyl acetals include polyvinyl acetaldehyde acetal, polyvinyl butyraldehyde acetal and polyvinyl sodium o-sulfobenzaldehyde acetal. Other useful colloid binding agents which can be used include the poly-N-vinyllactams of Bolton US. Patent 2,495,918, various polysaccharides, e.g., dextran, deXtrin, etc., the hydrophilic copolymers in Shacklett US. Patent 2,833,650, hydrophilic cellulose ethers and esters, and polymers of acrylic and Also, it has been found practical to treat silver halide layers on a base material in the essential absence of a binder, e.g., by chemical or vacuum deposition.

The emulsions may optionally contain any of the usual adjuvants customarily employed in silver halide systems so long as they do not interfere with the adsorption and complexing action of the essential ingredient of the invention.

The emulsions can be coated on any suitable support, e.g., cellulose esters, cellulose mixed esters; superpolymers, e.g., polyvinyl chloride (co) vinyl acetate, polyvinyl acetals, butyrals; polystyrene; polyamides, e.g., polyhexamethylene adipamide, polyesters, e.g., polycarbonates, polyethylene terephthalate, polyethylene terephthalate/isophthalate, esters formed by condensing terephthalic acid and its derivatives, e.g., dimethyl terephthalate with propylene glycol, diethylene glycol, tetramethylene glycol, cyclohexane 1,4 dimethanol (hexahydro-p-Xylene dialcohol); paper, metal, glass, etc.

As disclosed earlier, the desirable concentration of the selected organic compound depends on many factors such as the size and solubility of the organic compound, the nature of its reaction with silver halide, the size and nature of the silver halide crystals, the presence of other materials Which may react with or be adsorbed to the surface or" the silver halide, etc. In Example I below, a number of organic compounds are disclosed which were tested in a dispersion of silver halide crystals wherein the average grain size was 0.35 (micron) in diameter, therefore about 0043 in volume, assuming cubic grains. The silver halide comprised 70 mole percent silver chloride and 30 mole percent silver bromide, with a specific density of about 5.7 g./cc. or 5.7x l0 g./n The Weight per individual crystal or grain is 0.043/13X5JX1012 g./ =0.25 X 1() g. Assuming a molecular weight of 157 for the mixed AgCl-AgBr crystals, and dividing this number by the weight per grain, gives 157 g./mole-:0.25 g. =6.3 10 grains/mole. The area of a cubic grain of 0.35;. diameter=6 .35 =.74p. which, multiplied by the 63x10 grains per mole, gives a molar surface area of 4.6 10 or 4-.6 10 square angstroms.

A particularly preferred organic nitrogen-containing compound is S-nitrobenzimidazole. Assuming that a single molecule of this compound could occupy an area of 30 square angstroms, it would require 1.5 10 molecules to occupy a molar surface area of silver halide. With a molecular weight of 163, this would require to just cover the surface of one mole of the silver halide crystals. In Example I it required 1.0 10- g. of 5-nitrobenzimidazole to insolubilize 2.5 X l()- g. of silver halide of average molecular weight 157. Therefore according to experimental data (test tube results) it would require of S-nitrobenzimidazole to insolubilize one mole of the silver halide. More significantly, as disclosed in Example VII in a photographic emulsion coated on a film base support, it was found that 2.7 g. of S-nitrobenzimidazole per mole of silver halide gave optimum results. This compares somewhat more closely with the theoretically determined quantity required to cover the silver halide surface.

As shown in various examples below, e.g., Example II, elements suitable for this novel process can be prepared by bathing a photographic film in a solution of an appropriate organic compound. In this embodiment, the silver halide crystals near the surface of the coated emulsion stratum are in contact with a higher concentration of the organic compound. Crystals farther from the surface, are treated with less of the organic compound, and, if the rate of diffusion is sufficiently slow, there may be considerably less of the organic compound (even approaching zero) reacting with the lower than with the surface silver halide crystals. In such elements, satisfactory results might be obtained with only a fraction, e.g., one-half, of the amount of the organic compound theoretically calculated as required to just cover the surface of a mole of the silver halide crystals.

The invention will be further illustrated by but is not intended to be limited to the following examples.

EXAMPLE I Tests A and B have been described earlier as procedures whereby one can determine whether or not a given organic nitrogen-containing compound is suitable for use according to the process of this invention. Several of the compounds which were indicated as suitable according to the screening procedures of both tests, have been incorporated into actual photographic coatings and good results have been obtained. Below there are listed a number of organic nitrogen-containing compounds which were tested in this manner including three of the compounds which have been found to be inoperable. Of the unsuitable compounds shown, p-nitrophenol failed to produce the insolubility required of Test A and was therefore not subjected to further testing. Two other nitrogen-containing compounds 3 methyl 4 (m-nitrophenyl)-5-pyrazolone and carbazole, produced the required insolubility of Test A but failed to meet the requirements of solubilization according to Test B. Since it Was impracticable to perform complete photographic experiments with each and every compound screened according to Tests A and B, a simulated photographic test was devised and will be designated as Test C. It is noted that there is complete testing correlation in that any compound which was found suitable according to the photographic test to be described in the next paragraph was also found suitable according to Tests A and B.

TEST C A 0.5 ml. portion of the insolubilized dispersion prepared in Test A under safelight conditions is placed in a 12 x 75 mm. Pyrex test tube three inches from a No. 2 refiectofiood lamp. This insolubilized dispersion is exposed to the lamp for up to 10 minutes. A control consisting of another 0.5 ml. portion of the insolubilized silver halide dispersion from Test A is taken under safelight conditions. Two-tenths of a milliliter of 10% aqueous sodium thiosulfate is added to each of the dispersion samples taken and compared under safelight conditions. Any reduction in turbidity of the dispersion exposed to 7 the refiectorllood lamp compared to the unexposed control after treatment with aqueous sodium thiosulfate solution shows that photosolubilization occurs.

Tests A, B and C were all conducted using Silver Halide Dispersion I, the preparation of which was given immediately following the description of the procedure for Test B. To determine an approximate minimum concentration of the organic compound required to efiect insolubilization of silver halide in the presence of an aqueous solution of sodium thiosulfate, the qualitative procedure of Test A was repeated in a more quantitative manner, using a ripened, Washed and redispersed (but not chemically sensitized) gelatino-silver chlorobromide emulsion as described in Example I of assignees copending application, Nottorf, U.S. Serial No. 94,989, filed March 13, 1961. This emulsion is designated in the table below as Dispersion II and was made as follows: A lithographic emulsion having a silver halide composition of 30 mole percent AgBr and 70 mole percent AgCl and having 20 grams of gelatin present per mole of silver halide for the .steps of precipitation and ripening was freed of unwanted,

soluble, by-product salts by a coagulation and wash procedure as taught in Waller et al. US. Patent 2,489,341, wherein the silver halide and most of the gelatin were coagulated by an anionic wetting agent, sodium lauryl sulfate, using an acid coagulation environment. Following the washing step, the emulsion coagulate was redispersed in water together with 47 grams of additional bulking gelatin.

grain size, silver halide composition, etc., it is desirable to use conventional photographic dark room handling of the element prior to the image exposure. During the fixing step, the treated element is converted into an exact reproduction (i.e., direct positive silver halide image which is converted to a corresponding silver image in the second developing step) of the original transparency. After brief washing in water and drying, it is satisfactory for use in any application where an exact reproduction is desired, e.g., in the graphic arts field, for a projection transparency, etc.

No silver halide image is obtained when a similarly prepared and exposed sample is bathed in acid rather than the highly alkaline sodium thiosulfate solution described above.

EXAMPLE III Example II was repeated except that for the S-nitrobenzimidazole of that example a solution of 5-nitroindazole of the same concentration was used to give similar results to those described in Example II.

EXAMPLE IV Example II was repeated except that 2-methylbenzothiazole ethiodide was substituted for the S-nitro benzimidazole and a small quantity of potassium bromide was added to the aqueous alkaline sodium thiosulfate solution. The results were similar to those obtained in Example II and III but the quality of the positive image was not as good.

Nitrogen Containing Compounds Active At High pH Test Results with Dispersion I Gms. Compounds to Insolubilize Compound Insolnbiliza- Chemical Photosolu- Dispersion II tion Test A $olubilizabilization Containing tron Test B Test 0 mg. Silver Halide 1. 2-Methylbenzothiazole ethiodide Insoluble Soluble 0. 005 2. 2-Hydrazinobenzothiazolm. .do (lo 0.03 3. 2-A1ninobeuzothiazolc do do 0.02 4. Q-Arnino-G-methylbenzotl do do 0. O2 5. 2-Chlorobenzothiazole. .do do 0.015 6. 2-lvlethylthiazoline .d0 d0 0.04 7. s-Nitrobenzimidazole do do do 0.01 s. 3-A'Iethyl-4-(tn-nitrophenyD-fi-pyrazolone "do... Insolub1e Insoluble... 0. 03 9. 5-Nitroindazole do. Soluble Soluble 0. 01 10. Quinine bisulfite do. .....(i do 0.03 11. Carbazole Insolubie Insolubl 0.003

12. p-Nitrophenol.

EXAMPLE II EXAMPLE V A photographic element was prepared by coating an aqueous gelatin dispersion of siiver chlorobromide 70 mole percent silver chloride and mole percent silver bromide) on a film base prepared as described in Example IV of Alles US. Patent 2,779,684. The dispersion had a ratio of silver halide to gelatin of 28:1 by weight and was coated at a pH of 6 of a rate of 116 milligrams of silver halide per square decimeter. After drying, the element was bathed for about 15 seconds in a solution of equal parts water and ethyl alcohol containing 5 grams of S-nitrobenzimidazole and 20 grams of sodium hydroxide per liter. The dried element was then exposed behind a photographic transparency for 10 seconds to the radiation from a General Electric 2A photoflood lamp at a distance of about 6 inches. The exposed element was then immersed in a 0.5 M medium hydroxide solution containing 64 grams of sodium thiosulfate per liter for /2 to 1 minute resulting in removal of the silver salt in the exposed areas. Subsequently the fixed film was rinsed briefly in water and bathed in a rapid, fogging photographic developer solution comprising 1-phenyl-4-methyl- 3-pyrazolidone and hydroquinone as reducing agents to which there had been added potassium iodide. All of the above operations were carried out in ordinary fluorescent room illumination. Where a more sensitive product is prepared by appropriate selection of such factors as Example II was repeated execept that 2-amino-6-methylbenzothiozole was substituted for the S-nitrobenzamidazole of the example. In this case either an acid or alkaline sodium thiosulfate solution gave results similar to Example II.

EXAMPLE VI EXAMPLE VII An emulsion as described in Dispersion II, Example I was redispersed in a 5% gelatin solution which contained 47 g. gelatin per mole of the silver halide. A pH 6.0:.1 was maintained while dispersing 10 min. at F. The emulsion was brought to 2320 g. by addition of water and the temperature adjusted to F. From an alkaline solution, 2.7 grams of S-nitrobenzimidazole was added per mole of silver halide. Dimethylolurea hardener was added and the emulsion was diluted With water to a total Weight of 2334 g. per mole of silver halide. This emulsion, with a pH raised to 10.6 by the above additions, was applied at a coating weight of 46 mg. of silver per square decimeter on 0.004 inch thick polyester photographic film base as described in Example 11. The coating was dried and, after imagewise exposure to white light, was then treated with an aqueous sodium thiosoulfate solution which was 0.75 N in sodium hydroxide to yield a positive silver halide image. The image was intensified by flashing with white light followed by chemical reduction to a metallic silver image.

The silver halide photosoluble elements of this invention differ from conventional silver halide emulsions containing antifogging agents in that the insolubilizing compounds used in the photosoluble elements are present in substantially greater than fog-inhibiting amounts. Such latter amounts include the maximum quantity which provides low fog without serious loss in speed and photographic quality. For this reason it is not practical to use photosoluble elements in place of ordinary silver halide photographic materials. When photosoluble elements are exposed and processed normally, development proceeds slowly and incompletely to give a negative silver image having much less speed and lower density. In addition, fixing is slower and may be incomplete for practical fixing times. Thus, photosoluble elements require longer conventional processing times and give slower speed, inferior quality images when compared to ordinary silver halide photographic elements.

The novel photographic compositions of this invention have numerous advantages. A primary advantage is the simplicity of their preparation. They can be exposed and processed to images under ordinary room light conditions.

The photographic processes applicable to the compositions and elements of the invention likewise have advantages over previously known systems based on selective reduction of exposed silver halide for forming either direct positive or negative images without restorting to the special effects and sensitizing procedures previously used for preparing such images. In addition, since image formation does not require selective reduction, this present process is not limited to the use of certain photographic developing agents but may be accomplished by using a wide range of reducing agents. Many such compounds are of very low cost and can be used to form images of much higher covering power than customary, thus effecting important economies in processing, as well as greatly increasing the efficiency of the silver image with a resultant increase in sensitivity.

Another advantage of the compositions and elements of this invention is that they may be processed to form silver halide or silver images without the requirement of special equipment but instead conventional equipment and apparatus can be used. A further advantage is that the processes can be carried out successfully by photographic technicians and photographers of ordinary skill. A still further advantage is that the processes utilize conventional reducing agents, e.g., developers and fixing agents. Still additional advantages will be apparent from the above description of the invention.

I claim:

1. A photographic silver halide composition having a pH between 8 and 13 and before exposure to actinic radiation comprising silver halide crystals having associated therewith in substantially greater than fog-inhibiting amounts a hererocyclic nitrogen compound having in the state of its free base at least one intracyclic radical represented by the formula at least one of the bonds being attached to intracyclic carbon, any divalent atom of the compound that is attached to more than one atom being in the heterocyclic ring and whose silver salt is less soluble in water than untreated silver chloride; said heterocyclic nitrogen compound being present in such an amount, in terms of the ratio of its weight to the surface area of the silver halide crystals, that when admixed in such ratio with an aqueous silver chlorobroanide dispersion, 70/30 mole percent, gelatin dispersion containing 0.29 mg. of Ag in /2 ml., and said silver chlorobromide dispersion is treated with 10% by weight aqueous sodium thiosulfate, so that the resulting mixture contains 0.29 mg. of Ag and mg. of sodium thiosulfate, at least three times the amount of silver chlorobromide remains undissolved as compared with a similar dispersion successively treated with 5% by weight aqueous sodium hypochlorite and 10% by weight aqueous sodium thiosulfate, so that the resulting mixture contains 0.29 mg. of Ag, 25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate, after vigorous agitation of both dispersions for 30 seconds at 25 C.

2. A photographic element comprising a support and a layer having a pH between 8 and 13 and before exposure to actinic radiation comprising silver halide crystals having associated therewith in substantially greater than fog-inhibiting amounts a heterocyclic nitrogen compound having in the state of its free base at least one intracyclic radical represented by the formula at least one of the bonds being attached to intracyclic carbon, any divalent atom of the compound that is attached to more than one atom being in the hetereocyclic ring and whose silver salt is less soluble in water than untreated silver chloride; said heterocyclic nitrogen compound being present in such an amount, in terms of the ratio of its weight to the surface area of the silver halide crystals, that when admixed in such ratio with an aqueous silver chlorobromide dispersion, 70/30 mole percent, gelatin dispersion containing 0.29 mg. of Ag in /2 ml., and said silver chlorobromide dispersion is treated with 10% by weight aqueous sodium thiosulfate, so that the resulting mixture contains 0.29 mg. of Ag and 100 mg. of sodium thiosulf-ate, at least three times the amount of silver chlorobromide remains undissolved as compared with a similar dispersion successively treated with 5% by weight aqueous sodium hypchlorite and 10% by weight aqueous sodium thiosulfate, so that the resulting mitxure contains 0.29 mg. of Ag, 25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate, after vigorous agitation of both dispersions for 30 seconds at 25 C.

3. An element according to claim 2 wherein said heterocyclic nitrogen compound contains at least one negative group conjugated with the intracyclic group.

4. An element according to claim 2 wherein said heterocyclic nitrogen compound contains at least one nitro group conjugated with the intracyclic group.

5. An element according to claim 2 wherein said layer is composed essentially of said silver halide crystals.

6. A photographic element comprising a support bearing a water-permeable organic colloid layer having a pH between 8 and 13 and before exposure to aotinic radiation comprising silver halide crystals having associated therewith in substantially greater than fog-inhibiting amounts a heterocyclic nitrogen wmpound having in the state of its free base at least one intracyclic radical represented by the formula at least one of the bonds being attached to intracyclic carbon, any divalent atom of the compound that is attached to more than one atom being in the heterocyclic ring and whose silver salt is less soluble in water than untreated silver chloride; said heterocyclic nitrogen compound being present in such an amount, in terms of the ratio of its Weight to the surface area of the silver halide crystals, that when admixed in such ratio with an aqueous silver chlorobromide dispersion, 70/30 mole percent, gelatin dispersion containing 0.29 mg. of Ag in /2 ml., and said silver chlorobromicle dispersion is treated with 10% by weight aqueous sodium thiosulfate, so that the result- 1g mixture contains 0.29 mg. of Ag and 100 mg. of sodium thiosulfate, at least three times the amount of silver chlorobromide remains undissolved as compared with a similar dispersion successively treated with 5% by weight aqueous sodium hypochlorite and 10% by Weight aqueous sodium thiosulfate, so that the resulting mixture contains 0.29 mg. of Ag, 25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate, after vigorous agitation of both dispersions for 30 seconds at 25 C.

7. An element according to claim 6 wherein said colloid is gelatin and said silver halide is silver chlorobromide.

8. An element according to claim 6 wherein said colloid is gelatin and said silver halide is silver chlorobromide, and the gelatin/silver halide ratio is from 3:1 to 1:30 by weight.

9. A photographic element according to claim 6 wherein said heterocyclic nitrogen compound is S-nitrobenzimidazole.

i 2 10. A photographic element according to claim 6 wherein said heterocyclic nitrogen compound is 2-arnino-6-methylbenzothiazole.

11. A photographic element according to claim 6 wherein said heterocyclic nitrogen compound is Z-methylbenzothiazole ethiodide.

References Cited in the file of this patent UNITED STATES PATENTS 2,131,038 Brooker et al Sept. 27, 1938 2,214,446 Albers et al. Sept. 10, 1940 2,728,667 Knott et a1. Dec. 27, 1955 3,008,829 Clementi et a1. Nov. 14, 1961 3,046,130 Dersck et a1. July 24, 1962 3,063,837 Lassig et al. Nov. 13, 1962 3,080,230 Haydn et a1. Mar. 5, 1963 OTHER REFERENCES 

1. A PHOTOGRAPHIC SILVER HALIDE COMPOSITION HAVING A PH BETWEEN 8 AND 13 AND BEFORE EXPOSURE TO ACTINIC RADIATION COMPRISING SILVER HALIDE CRYSTALS HAVING ASSOCIATED THEREWITH IN SUBSTANTIALLY GREATER THAN FOG-INHIBITING AMOUNTS A HEREOCYCLIC NITROGEN COMPOUND HAVING IN THE STATE OF ITS FREE BASE AT LEAST ONE INTRACYCLIC RADICAL REPRESENTED BY THE FORMULA 